Bulk superhard B-C-N nanocomposite compact and method for preparing thereof

ABSTRACT

Bulk, superhard, B-C-N nanocomposite compact and method for preparing thereof. The bulk, superhard, nanocomposite compact is a well-sintered compact and includes nanocrystalline grains of at least one high-pressure phase of B-C-N surrounded by amorphous diamond-like carbon grain boundaries. The bulk compact has a Vicker&#39;s hardness of about 41-68 GPa. It is prepared by ball milling a mixture of graphite and hexagonal boron nitride, encapsulating the ball-milled mixture, and sintering the encapsulated ball-milled mixture at a pressure of about 5-25 GPa and at a temperature of about 1000-2500 K.

STATEMENT REGARDING FEDERAL RIGHTS

[0001] This invention was made with government support under ContractNo. W-7405-ENG-36 awarded by the U.S. Department of Energy. Thegovernment has certain rights in the invention.

FIELD OF THE INVENTION

[0002] The present invention relates generally superhard materials andmore particularly to a superhard compact of nanocrystalline grains of atleast one high-pressure phase of B-C-N embedded in a diamond-likeamorphous carbon matrix and to a method for preparing the superhardcompact.

BACKGROUND OF THE INVENTION

[0003] Superhard materials have a Vickers hardness (Hv), i.e. anindentation hardness, of at least 40 GPa and are widely used asabrasives for drilling, cutting, and other machining applications.Superhard materials often include boron, carbon, nitrogen and oxygenbecause these light elements have a small atomic radius and form strongand directional covalent bonds that produce tight, three-dimensionalnetworks with extreme resistance to external shear.

[0004] Diamond is the hardest superhard material currently known, withan H_(V) of about 70-100 GPa. However, the actual performance of diamondas an abrasive is somewhat limited. Diamond is an unsuitable abrasivefor machining ferrous alloys and has limited applications for high-speedcutting because it is converted into graphite in the presence of oxygenat temperatures over 800° C.

[0005] Cubic BN (cBN) is another important superhard material. While cBNis widely used for machining fully hardened steels and exhibits muchbetter thermal stability than diamond, it is only about half as hard(H_(V)=45˜50 GPa) as diamond.

[0006] Superhard materials for industrial use are often in the form ofsintered polycrystalline composites that incorporate microcrystallinegrains of diamond or cubic boron nitride. The grains of this compositeare tens to hundreds of micrometers in size, and usually includevacancies, dislocations, and other imperfections that multiply andpropagate to form microcracks within individual crystals of a grain, andalso along grain boundaries. As the microcracks grow, the materialsdeform and fracture.

[0007] Recently, a new class of materials known as superhardnanocomposites has been reported. Superhard nanocomposites containsuperhard nanocrystalline grains embedded in an amorphous matrix. Theamorphous matrix provides amorphous grain boundaries that absorbvacancies and dislocations, reduces the surface energy and residualstress among the grains, and permits the relaxation of mismatchesbetween adjacent grains of different phases. While a number of superhardnanocomposites have been reported, no superhard nanocomposite bulkcompact having the Vickers hardness of diamond has yet been prepared.Thus, there remains a need for a superhard nanocomposite compact withimproved hardness, strength, and performance.

[0008] Therefore, an object of the present invention is to provide abulk superhard nanocomposite compact with a high Vickers hardness.

[0009] Another object of the invention is to provide a method forpreparing a bulk superhard nanocomposite compact with a high Vickershardness.

[0010] Additional objects, advantages and novel features of theinvention will be set forth in part in the description which follows,and in part will become apparent to those skilled in the art uponexamination of the following or may be learned by practice of theinvention. The objects and advantages of the invention may be realizedand attained by means of the instrumentalities and combinationsparticularly pointed out in the appended claims.

SUMMARY OF THE INVENTION

[0011] In accordance with the purposes of the present invention, asembodied and broadly described herein, the present invention includes asuperhard nanocomposite compact. The compact consists essentially ofnanocrystalline grains of at least one high-pressure phase of B-C-Nsurrounded by amorphous diamond-like carbon grain boundaries.

[0012] The invention also includes a process for preparing a bulksuperhard nanocomposite compact consisting essentially ofnanocrystalline grains of at least one high-pressure phase of B-C-Nsurrounded by amorphous, diamond-like grain boundaries. To prepare thecompact, a mixture of graphite and hexagonal boron nitride isball-milled. The ball-milled mixture contains amorphous and/ornanocrystalline graphitic carbon and boron nitride. The ball-milledmixture is encapsulated and sintered at a pressure of about 5-25 GPa andat a temperature of about 1000-2500 K.

[0013] The invention is also a bulk, superhard nanocomposite compactprepared by the process of ball-milling a mixture of graphite andhexagonal boron nitride until the mixture is transformed into amorphousand/or nanocrystalline graphitic carbon and boron nitride. The ballmilled mixture is encapsulated and sintered at a pressure of about 5-25GPa and at a temperature of about 1000-2500 K.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The accompanying drawings, which are incorporated in and form apart of the specification, illustrate the embodiment(s) of the presentinvention and, together with the description, serve to explain theprinciples of the invention. In the drawings:

[0015]FIG. 1 shows a high magnification SEM image of the precursormaterial used to prepare the bulk, superhard nanocomposite compact ofthe invention;

[0016]FIG. 2 shows an x-ray diffraction pattern of the precursormaterial of FIG. 1;

[0017]FIG. 3 shows and a Raman spectrum of the precursor material ofFIG. 1;

[0018]FIG. 4 shows diffraction patterns plotted as intensity versus2-Theta Angle;

[0019]FIG. 5 shows synchrotron x-ray patterns in the energy dispersivemode for three compacts of the invention;

[0020]FIG. 6 shows a synchrotron XRD pattern plotted as intensity versusd-spacing for a compact of the invention prepared at 20 GPa and 1900 C;

[0021]FIG. 7 shows a high-resolution transmission electron microscopy(HRTEM) image of a compact of the invention; and

[0022]FIG. 8 shows an electron energy-loss spectroscopy (EELS) spectrumfor amorphous, ball-milled starting material and an EELS spectrum for acompact of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention includes a superhard B-C-N nanocompositecompact and a method for preparing the compact. The compact includesnanocrystalline grains of at least one high-pressure B-C-N phaseembedded in a diamond-like amorphous matrix. The practice of theinvention can be further understood with the accompanying figures.Similar or identical structure is identified using identical callouts.

[0024] The compact is produced by first preparing a ball-milled mixtureof graphite and hexagonal boron nitride (hBN). A tungsten carbide vialand tungsten carbide milling balls were used for the ball millingprocedure. FIG. 1 shows a high magnification scanning electronmicroscope (SEM) image of the ball-milled powdered mixture after 34hours of ball milling. As the SEM image shows, the mixture is dark, hasa grain size less than 0.1 micron, and does not appear to have acrystalline morphology.

[0025]FIG. 2 includes two x-ray diffraction spectra. The upper spectrumis of the mixture of graphite and hexagonal boron nitride before ballmilling. The lower spectrum is of the ball-milled mixture after 34 hoursof ball milling. As FIG. 2 shows, the ball-milled mixture appears to beamorphous.

[0026]FIG. 3 shows three Raman spectra. The upper spectrum is ofhexagonal boron nitride (hBN), the middle spectrum is of graphite, andthe lower spectrum is of the ball-milled mixture of graphite and hBN.The lower spectrum includes a peak at 1350 cm⁻¹ (a defect/disorder peak)and a peak at 1580 cm⁻¹ that is assigned to a graphitic phase.Importantly, the lower spectrum suggests that along with a graphitephase, the ball-milled mixture also includes grains of nanocrystallineparticles and the intensity ratio between the 1350 cm⁻¹ peak and the1580 cm⁻¹ peak indicates that the grain size is about 2-3 nanometers(nm).

[0027] A sample of the ball-milled powder having the lower Ramanspectrum of FIG. 3 was encapsulated in a cylindrical platinum capsuleand compressed using a multi-anvil to a pressure of about 5-25 GPa. Atthis elevated pressure, the encapsulated powder was sintered at atemperature of about 1000-2500 K for 2-120 minutes. After the sinteringperiod, the capsule was brought to room temperature and decompressed toambient pressure. The compact was removed from the capsule and the endsof the compact were polished with a diamond abrasive. The resultingpolished compact was a well-sintered cylindrical bulk compact having aheight of about 1.5-mm and a diameter of about 1.2-mm. These dimensionsare a reflection of the dimensions of the capsule used. Obviously,compacts of different sizes and shapes depend on the size and shape ofthe capsule and the cell assembly used. A larger capsule and cellassembly would require a larger sample size and result in a largercompact. Likewise, smaller capsules and cell assemblies could also beused to prepare smaller compacts.

[0028] Several examples of the bulk compact of the invention wereprepared according to the conditions summarized in Table 1 below. TABLE1 Precursor Sintering Sintering Vickers powder Pressure temperature Timehardness Entry composition (GPa) (K) (minutes) (GPa) Color 1 BCN 20 13002 Black 2 BCN 20 2100-2400 10 Light yellow 3 BC₂N 6-8 1500 120 Black 4BC₂N 10 2000 5 Black 5 BC₂N 15 1800 5 50 6 BC₂N 15 2000 5 41 Black 7BC₂N 16 2100 5 50 Gray-white 8 BC₂N 20 2200 5 62 Light yellow 9 BC₂N 252130 10 Light yellow 10 BC₂N 25 2300 60 Light yellow 11 BC₄N 20 2300 568 brown

[0029] As Table 1 shows, the compacts varied in color. Some weretranslucent, while others were opaque. Some were black (entries 1, 3, 4,and 6) while others were gray-white (entry 7), brown (entry 11), andlight yellow (entries 2, 8, 9, and 10. The color seems to be dependenton the relative amount of graphite, the pressure, and the sinteringtemperature).

[0030] The Vickers hardness for several of them (entries 5, 6, 7, 8, and11) were measured and determined to be in the range of about 41-68 GPa.The Hv for any particular compact of the invention appears to bedependent upon the precise composition of the precursor powder and onthe synthesis conditions. Three precursor powder compositions were used.Entries 1 and 2 (BCN) employed a composition of a 1:1 molar ratio ofgraphite:hBN (i.e. 1 part graphite and 1 part hBN). Entries 2-9 (BC₂N)employed a composition 2:1 molar ratio of graphite:hBN (i.e. 2 partsgraphite and 1 part hBN). Entry 11 (BC₄N) employed a powder compositionof a 4:1 molar ratio of graphite:hBN (i.e. 4 parts graphite and 1 parthBN. Pressures varied from about 6 GPa to about 25 GPa, sinteringtemperatures varied from about 1300 K (entry 1) to about 2400 K (entry2), and sintering times varied from 2 minutes (entry 1) to about 120minutes (entry 3). The preparation of several of these compacts is nowdescribed.

EXAMPLE 1

[0031] The compact of entry 2 was synthesized as follows. About 5 gramsof a ball milled mixture of a 1:1 molar ratio of graphite:hBN wereprepared. About 3 mm³ of the ball-milled mixture was placed into aplatinum capsule. Using a split-sphere multi-anvil press, theencapsulated mixture was subjected to a pressure of about 20 GPa andthen sintered at a temperature of about 2100-2400 K for about 10minutes. The resulting compact was light yellow in color.

EXAMPLE 2

[0032] The compact of entry 5 was synthesized as follows. About 3 mm³ ofthe ball-milled mixture described in Example 1 was placed into aplatinum capsule. Using the anvil press of example 1, the encapsulatedmixture was subjected to a pressure of about 15 GPa, and then sinteredat a temperature of about 2100 K for about 5 minutes. The resulting bulkcompact had a measured Vickers hardness was 50 GPa.

EXAMPLE 3

[0033] The compact of entry 8 was synthesized as follows. About 3 mm³ ofthe ball-milled mixture described in Example 1 was placed into aplatinum capsule. Using the anvil press of Example 1, the encapsulatedmixture was subjected to a pressure of about 20 GPa, and then sinteredat a temperature of about 2200 K for about 5 minutes. The resulting bulkcompact of the invention was light yellowish in color, translucent, andhad a measured Vickers hardness of 62 GPa.

EXAMPLE 4

[0034] The compact of entry 9 was synthesized as follows. About 3 mm³ ofthe ball-milled mixture described in Example 1 was placed into aplatinum capsule. Using the anvil press of Example 1, the encapsulatedmixture was subjected to a pressure of about 25 GPa, and sintered at atemperature of about 2130 K for about 10 minutes. The resulting bulkcompact of the invention was light yellow in color.

EXAMPLE 5

[0035] The compact of entry 10 was synthesized as follows. About 3 mm³of the ball-milled mixture described in Example 1 was placed into aplatinum capsule.

[0036] Using the anvil press of Example 1, the encapsulated mixture wassubjected to a pressure of about 25 GPa, and sintered at a temperatureof about 2300 K for about 60 minutes. The resulting bulk compact of theinvention was light yellow in color.

EXAMPLE 6

[0037] Compact number 11 was synthesized as follows. A mixture of a 4:1molar ratio of graphite:hBN was prepared. About 3 mm³ of the ball-milledmixture was placed into a platinum capsule. Using the anvil press ofExample 1, the encapsulated mixture was subjected to a pressure of about20 GPa and sintered at a temperature of about 2200 K for about 5minutes. The resulting bulk compact of the invention was brownish andtranslucent, with a measured Vickers hardness was 68 GPa.

[0038] The microstructure and composition of the compact of theinvention was probed using a variety of techniques. While opticalmicroscopy and scanning microscopy were relatively uninformative, thegranular structure of the compact of the invention was revealed usingthe Advanced Photon Source (APS) at Argonne National Laboratory, whichprovided monochromatic synchrotron x-ray diffraction in angle dispersivemode. The compact was interrogated using a narrow (5×7 μm²), collimatedX-ray beam (λ=0.4146 Å). The x-rays by the compact were collected usingan image plate in angle-dispersive mode to cover a 2-Theta (2Θ) anglerange up to 32 degrees, which corresponds to a minimum d-spacing of 0.77Å. Changing the position of the beam spot on the compact had no effecton the diffraction pattern, which indicated that the sample washomogeneous in structure and composition.

[0039]FIG. 4 shows X-ray diffraction patterns that are plotted asintensity versus 2-Theta Angle for the BC₂N bulk compact of theinvention synthesized at a pressure 20 GPa and a sintering temperatureof 2200 K. The upper diffraction pattern was obtained when the compactwas rocked with an amplitude of 5 μm. The middle diffraction pattern isfor the non-rocking, stationary compact, and the lower pattern is astandard diffraction pattern of cerium oxide (CeO₂), which is includedin order to indicate the resolution of the x-ray diffraction instrument.As FIG. 4 shows, the peaks of the middle pattern are about 5-6 times asbroad as the peaks of the lower pattern. As the upper pattern shows, thepeaks broadened even more (8-10 times as broad as the lower pattern)when the sample was rocked. From these observations, it was concludedthat the compact includes nanocrystalline grains. Using Scherrer'sequation, the grain size was estimated at about 4-8 nm.

[0040] The major diffraction peaks shown in FIG. 4 for the compact ofthe invention are consistent with a face-centered-cubic (fcc)zinc-blende (ZnS) structure with a unit cell parameter a=3.595(7) A.This unit cell dimension lies between diamond (a=3.567 Å) and cBN(a=3.616 Å), and is in close agreement with the unit cell parameterreported by E. Knittle et al. in “High Pressure Synthesis,Characterization, and Equation of State of Cubic C-BN Solid Solutions,”Phys. Rev. B. vol. 51,1995, pp.12149-12156; by T. Komatsu et al. in“Creation of Superhard B-C-N Heterodiamond Using Shock Wave CompressionTechnology,” J. Mater. Processing Technology, vol. 85,1999, pp. 69-73;and by W. Utsumi et al., in “In situ X-Ray and TEM Observations on thePhase Transitions of BC₂N Under Static Pressures,” Proceedings ofAIRAPT-18, Beijing, 2001, p. 186. From these papers, it appears that E.Knittle et al., T. Komatsu et al. and W. Utsumi et al. were unable toproduce the well-sintered, bulk superhard nanocomposite compact of thepresent invention.

[0041]FIG. 5 shows synchrotron x-ray diffraction patterns in the energydispersive mode for three compacts of the present invention. The compactthat produced the top x-ray diffraction pattern was sintered at 15 GPaand 2000 K. The compact that produced the middle x-ray diffractionpattern was sintered at 16 GPa and 2100 K. The compact that produced thebottom x-ray diffraction pattern was sintered at 20 GPa and 2200 K. Eachpattern includes the <111> pc and <220> pc peaks of the fcc lattice.Compacts of the invention prepared at 20 GPa were about 15-20% harderthan those prepared at 15-16 GPa. Compacts that were prepared at stilllower pressures produced x-ray diffraction patterns that exhibitedadditional peaks and apparent peak splitting that was most noticeablefor the <200> pc peak. This may suggest the existence of a superlattice,lower symmetry, or an additional phase. Generally, compacts prepared athigher pressures appear to have higher symmetry.

[0042]FIG. 6 shows the synchrotron XRD pattern, plotted as intensity inarbitrary units versus d-spacing in angstroms, of a compact prepared at20 GPa and 2200 K. The top left inset shows that the <111> peak can befitted by a sum of two curves; a broadened crystalline peak (curve A)and an amorphous hump (curve B). The plus (+) indicates the dataobserved and the dark solid line is the curve calculated from thefitting of curve A with curve B.

[0043]FIG. 7 shows high-resolution transmission electron microscopy(HRTEM) image of a BC₂N compact of the present invention synthesized ata pressure of 20 GPa and a sintered at a temperature of 2200 K for 5minutes. The HRTEM image confirms the presence of 3-8 nm nanocrystallinegrains with an average size of about 5 nm, which is consistent with thesynchrotron X-ray diffraction pattern. The zinc-blende fcc structure,apparent from FIG. 4, is confirmed by the electron diffraction patternshown in the top right inset of FIG. 7. FIG. 7 includes an enlargedimage of a grain. The enlarged image appears to include an apex of aregular icosohedron, a regular polyhedron with 20 triangular faces andfive-fold symmetry. Lines have been added to more clearly show thesefeatures. Also according to FIG. 7, the grain boundaries between thenanocrystalline grains appear to be amorphous.

[0044] The chemical composition and chemical bonding of individualgrains of the compact were determined using electron energy-lossspectroscopy (EELS), a powerful technique for obtaining local chemicalcomposition and chemical bonding information in materials composed oflight elements. Samples of the compact were prepared for EELS by anion-thinning process or by directly impacting the sample into finepowder. The results were the same for both sample preparation methods. Anarrow (3-4 nm) focused electron beam was used to probe the chemicalcomposition and bonding of individual nanocrystalline grains. FIG. 8shows an upper EELS spectrum for the amorphous, ball-milled startingmaterial and a lower EELS spectrum for a single nanocrystalline grain.The lower EELS spectrum includes the K-edges for B, C, and N, whichconfirms that the grain is composed of a single ternary B-C-N phaserather than a mixture of a diamond phase and a cBN phase. The upper EELSspectrum for the amorphous ball-milled starting material includes π*peaks at the K-edges for the B, C, and N. The appearance of these π*peaks in the amorphous starting material suggests the presence ofsp²-hybridized hexagonal ring fragments. The π* peaks of the EELSspectrum of the amorphous material do not appear in the EELS spectrum ofthe compact.

[0045] The chemical composition of the grain boundaries was examinedusing a combination of HRTEM and EELS. Unexpectedly, the grainboundaries are composed of amorphous, diamond-like carbon (DLC). DLC istypically produced by such methods as vacuum arc or pulsed laserdeposition, and has stimulated great interest because of its highhardness, chemical inertness, thermal stability, wide optical gap, andnegative electron affinity. It is believed that the bulk, superhard,nanocomposite compact of the invention is the first bulk, nanostructuredcompact reported with DLC grain boundaries, which are believed tocontribute significantly to the mechanical strength of the compact.

[0046] The enhanced fracture toughness of the compact of the inventionis likely due, at least in part, to the substantial absence of vacanciesand dislocations in the individual nanocrystalline grains, and also tothe difficulty of microcrack propagation through the amorphous grainboundaries separating the grains.

[0047] The effects of using a ball-milled amorphous material as theprecursor material were examined by preparing compacts from a differentprecursor material: a mixture of graphite and hexagonal boron nitride(hBN) that had not been subjected to ball milling. Compacts preparedwithout ball milling the mixture of graphite and hBN did not includenanocrystalline grains of BC₂N. Instead, these compacts includedsegregated phases of diamond and cBN. The presence of segregated phaseswas first suggested by optical microscopy, more strongly indicated byx-ray diffraction spectra that showed twin-peaks of all the major x-raydiffraction peaks, and finally confirmed by Raman spectra that showedthe characteristic peaks of diamond and cBN.

[0048] The invention also includes machining tools of the bulk superhardcompact of the invention. The compact could be used for drilling,cutting, puncturing, and other types of machining.

[0049] In summary, the invention includes a well-sintered, bulk,superhard, nanocomposite compact and a method for preparing the bulkcompact. The bulk compact includes nanocrystalline grains of at leastone high-pressure phase of B-C-N embedded in a diamond-like amorphouscarbon matrix. A variety of analytical techniques show that the bulkcompact contains nanocrystalline grains of B-C-N having a diamond-likestructure. The structure symmetry and Vickers hardness (Hv=50-73 GPa) ofthe bulk compact of the invention appear to increase with the pressureused to prepare the compact. The Vickers hardness of several examples ofthe bulk compact was higher than that for cBN (47 GPa, see T. Taniguchiet al. in “Sintering of cubic boron nitride without additives at 7.7 GPaand above 2000° C., J. Mater. Res., vol. 14, pp. 162-169, 1999) and forhBN single crystals (45-50 GPa, see Handbook of Ceramic Hard Materials,R. Riedel ed., pp. 104-139, Wiley-VCH Verlag GmbH, D-69469, Weinheim,2000) and were very close to the hardness of diamond (70-100 GPa) It isexpected that the compact of the invention is more stable at hightemperatures than diamond and that machining tools employing the compactof the invention will not react with ferrous metals during high-speedcutting.

[0050] The foregoing description of the invention has been presented forpurposes of illustration and description and is not intended to beexhaustive or to limit the invention to the precise form disclosed, andobviously many modifications and variations are possible in light of theabove teaching. Commercially available autofocus laser end effectors,for example, could be used instead of the laser end effectors describedherein.

[0051] The embodiment(s) were chosen and described in order to bestexplain the principles of the invention and its practical application tothereby enable others skilled in the art to best utilize the inventionin various embodiments and with various modifications as are suited tothe particular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

1-5. (canceled)
 6. A process for preparing a bulk, superhard,nanocomposite compact consisting essentially of nanocrystalline grainsof at least one high-pressure phase of B-C-N surrounded by amorphous,diamond-like carbon grain boundaries, comprising the steps of: (a) ballmilling a mixture of graphite and hexagonal boron nitride to produce amixture of amorphous and/or nanocrystalline graphitic carbon and boronnitride; (b) encapsulating the ball-milled mixture; and (c) sinteringthe encapsulated ball-milled mixture at a pressure of about 5-25 GPa anda temperature of about 1000-2500 K, thereby producing a bulk, superhardnanocomposite compact consisting essentially of nanocrystalline grainsof B-C-N surrounded by amorphous diamond-like carbon grain boundaries.7. The process of claim 6, wherein the ball milled mixture of graphitehexagonal boron nitride consists essentially of about 1-4 parts graphiteto about 1 part hexagonal boron nitride.
 8. The process of claim 7,wherein the ball milled mixture of graphite and hexagonal boron nitrideconsists essentially of about 1 part graphite to about 1 part hexagonalboron nitride.
 9. The process of claim 7, wherein the ball milledmixture of graphite and hexagonal boron nitride consists essentially ofabout 2 parts graphite to about 1 part hexagonal boron nitride.
 10. Theprocess of claim 7, wherein the ball milled mixture of graphite andhexagonal boron nitride consists essentially of 4 parts graphite toabout 1 part hexagonal boron nitride.
 11. The process of claim 7,wherein the encapsulated ball-milled mixture is sintered at a pressureof about 10-25 GPa and at a temperature of about 2000-2500 K.
 12. Theprocess of claim 7, wherein the encapsulated ball-milled mixture issintered at a pressure of about 15-25 GPa and at a temperature of about2000-2500 K.
 13. The process of claim 7, wherein the encapsulatedball-milled mixture is sintered at a pressure of about 16-25 GPa and ata temperature of about 2100-2500 K.
 14. The process of claim 7, whereinthe encapsulated ball-milled mixture is sintered at a pressure of about20-25 GPa and at a temperature of about 2000-2500 GPa.
 15. The processof claim 7, wherein the encapsulated ball-milled mixture is sintered ata pressure of about 20-25 GPa and at a temperature of about 2100-2400 K.16. The process of claim 7, wherein the encapsulated ball-milled mixtureis sintered at a pressure of about 20 GPa and at a temperature of about2000-2400 K.
 17. The process of claim 7, wherein the encapsulatedball-milled mixture is sintered at a pressure of about 25 GPa and at atemperature of about 2100-2300 K.
 18. The process of claim 6, whereinstep (b) comprises encapsulating the amorphous mixture in capsulecomprising platinum, gold, rhenium, or boron nitride.
 19. The process ofclaim 7, wherein said compact has a Vickers hardness of about 41-68 GPa.20. The process of claim 7, wherein said compact has a Vickers hardnessof about 50-68 GPa.
 21. The process of claim 7, wherein said compact hasa Vickers hardness of about 62-68 GPa.
 22. The process of claim 7,wherein said compact has a Vickers hardness of 68 GPa. 23-38. (canceled)39. A machining tool comprising a bulk, superhard, nanocomposite compactconsisting essentially nanocrystalline grains of B-C-N surrounded byamorphous diamond-like carbon grain boundaries.
 40. The tool of claim39, wherein said compact has a Vickers hardness of about 41-68 GPa. 41.The tool of claim 39, wherein said compact has a Vickers hardness ofabout 50-68 GPa.
 42. The tool of claim 39, wherein said compact has aVickers hardness of about 62-68 GPa.
 43. The tool of claim 39, whereinsaid compact has a Vickers hardness of 68 GPa.